Explain why cache not updating frequently

Hey all, just wondering if anyone has any pointers as to why cache of a particular blog does not update as frequently as it should.

The site in question is a WP blog, updated every day with new content, and it used to cache every day. Now it stays a week behind or so. The XML sitemap submits every day, and it pings services automatically...content regularly updated. So what would cause a lag in caching? PR: wait... I: wait... L: wait... LD: wait... I: wait...wait... Rank: wait... Traffic: wait... Price: wait... C: wait...